The present invention relates to storage of a gas in an underground cavern. In storage, a portion of the gas remains in the underground cavern to supply the necessary pressure to withdraw the stored gas. That portion is called cushion gas. This invention is particularly related to a method of underground gas storage in which a substitute gas is used as the cushion gas.
With natural gas, underground storage is a method by which a constant supply from a natural gas pipeline is combined with a variable demand for the natural gas for an economic advantage. Because the natural gas consuming market is seasonal and critically fluctuates with the severity of weather, it is common practice to store the gas transported by pipelines in some underground storage environment during the summer and then retrieve the gas from the storage during winter when market demand exceeds the rate deliverable from the pipeline. The environment for storage is usually provided by depleted gas or oil fields, porous and permeable aquifers as well as underground cavities, mined or developed in salt domes by fresh water leaching.
The pressures maintained in storage fields cycle between a maximum value after injection of the gas at high inventory and a minimum value corresponding to a low inventory reached at the end of winter withdrawal. The maximum pressure is limited by the mechanical integrity of the cap rock and surface equipment while the minimum pressure is limited by the required gas deliverability from the storage field and structural circumstances related to cavern stability. It is well known in storage operations that the gas deliverability which must be maintained during "peak day" and/or "last day" of the season critically depends upon the remaining gas inventory as measured by the prevailing pressure. In practical terms, this simply means that a certain amount of natural gas must be maintained in the storage reservoir at all times just to pressure it sufficiently to sustain the minimum gas deliverability. In salt cavern storage some minimum gas pressure is also necessary in order to maintain the volume and the shape of the cavity and to prevent the roof from collapsing.
The gas which must be committed to underground storage and held therein is called the "cushion gas". The gas which is injected and withdrawn each season is called the "top gas".
Depending upon the type of storage site, as much as 30% to 60% of the maximum amount stored could be the cushion gas. In natural gas storage, the use of natural gas as a cushion gas can be extremely expensive. Use of a cheaper substitute as the cushion gas has been developed in aquifer storage or depleted gas or oil field storage. Nitrogen gas, which currently costs much less than the cost of natural gas, has been used as a cushion gas. In underground storage reservoirs operated in porous environments the nitrogen cushion gas is directly injected into the reservoir. In typical porous rock it can coexist with the top gas without excessive, undesired mixing. The amount of the substitute inert cushion gas used is limited by its tendency to mix with the natural gas and dilute the heating value of the storage gas. In actual field operations where inert nitrogen is partially used as a substitute cushion gas, it has not exceeded 20% of the total cushion gas. By increasing the amount of inert nitrogen used as a cushion gas in natural gas storage, substantial savings can be achieved.
It is an object of the present invention to provide a method of storing a gas underground while using a different gas for a majority of the cushion gas without undesired mixing of the gases.
It is an advantage of the present invention that when an expensive gas is stored in an underground cavern, by using a less expensive substitute cushion gas, substantial cost savings can be realized.
According to the present invention, the cushion gas is separated from the top gas by a flexible, gas impervious membrane to prevent intermixing of the cushion gas with the top gas. The invention is most easily carried out in a mined or leached cavern such as a salt cavern having a single relatively large enclosed void space. One way of separating the cushion gas from the top gas is to divide the cavern into two portions by attaching a flexible membrane to the cavern wall about its periphery. The lower portion is filled with the cushion gas while the upper portion of the cavern is filled with the top gas. Another way of providing the gas separation is to install a pipe extending down into the cavern and attaching to the end thereof a large bladder that is filled with the cushion gas. Depending upon the quantity of natural gas in the cavern, the bladder will either expand or contract. Another way of separating the two gases includes lowering an uninflated balloon into the cavern inflating the balloon with the cushion gas and releasing the balloon in the cavern. This process is repeated until a sufficient quantity of inflated balloons have been deployed into the cavern to provide the necessary cushion. Multiple gases can be stored simultaneously by dividing the cavern into a plurality of separated portions.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in conjunction with the accompanying drawings.